Recently, considerable progress has been made in the generation of high power pulses for application to a wide-band impulse transmitter by charging up the semiconductor material of a transmission line and discharging it by rendering the material at an end of the line conductive with a pulsed laser beam. This causes a traveling wave to flow to that end of the line that can be coupled to an antenna. Thus the semiconductor material stores the energy and acts as a switch. Coaxial and microstrip lines have been used, but radial transmission lines produce pulses of higher voltage because the traveling waves converge at its center.
A radial transmission line device and a system for using it to generate radio frequency pulses in response to pulses of laser light is described in an article entitled "Monolithic, photoconductive impulse generator using a GaAs wafer" that was authored by people including the inventors of this invention and which appeared in the Jun. 17, 1991 issue of "Applied Physics Letters 58 (24)".
Various aspects of this known system are shown in FIGS. 1, 2 and 3 wherein a disk 2 of semiconductor material such as GaAs has an annular metal layer 4 ohmically bonded to one surface 5 by n or p doped region 6 of the disk 2 and an annular metal layer 8 ohmically bonded to the other surface 9 by an n or p doped region 10. The central openings of the annular metal layer 4 and the underlying doped region 6 are respectively indicated as 12 and 14. An inner conductor 16 of a coaxial line 18 is ohmically bonded to the center of the opening 20 in the annular layer 8 by an n or p type doped region 22. The outer conductor 24 of the coaxial line 18 is ohmically adhered to the annular metal layer 8 by soldering or the like as indicated at 26.
Generation of RF pulses is achieved by connecting a source 28 of DC charging pulses between the metal layers 8 and 4 and directing much shorter trigger pulses of laser light from a source 30 through the openings 12 and 14. In a matter of nanoseconds, the laser light renders the central portion of the wafer 2 conductive so that the capacitor formed by the disk 2 and the metal layers 4 and 8 is discharged through a load impedance 32 that is connected between the inner and outer conductors 16 and 24 of the coaxial line 18. This causes RF waves to travel inwardly to the center of the wafer and along the transmission line 18. The load 32 is typically a remotely located antenna.
One of the problems encountered with the device just described is the inefficient manner with which the optical energy from laser diodes made of GaAs material is transferred to the wafer disk of GaAs. This means that a larger array of laser diodes must be used in order to transfer a required amount of optical energy to the disk. The resulting increase in the threshold current for driving the array increases the rise time of the laser pulses thereby limiting the range of RF frequencies that can be produced. The increase in threshold current also reduces the laser pulse repetition frequency so as to reduce the RF output power that may be attained.
Furthermore the thermal and mechanical stability of the known device is such as to reduce its life when high bias voltage and a high pulse repetition frequency are employed to increase its output power.